KARACHI, Pakistan — A car bomb exploded outside a mosque on Sunday, killing at least 37 people and wounding another 141 in a neighborhood dominated by Shiite Muslims in the southern Pakistan city of Karachi.

No one has taken responsibility for the bombing, but Shiites Muslims have been increasingly targeted by Sunni militant groups in Karachi, Pakistan’s economic hub and site of years of police, sectarian and ethnic violence.

The bomb exploded outside a Shiite mosque as people were leaving evening prayers. Initial reports suggested the bomb was rigged to a motorcycle, but a top police official, Shabbir Sheikh, said later that an estimated 220 pounds of explosives was planted in a car.

Col. Pervez Ahmad, an official with a Pakistani paramilitary force called the Rangers, said a chemical used in the blast caught fire and spread the destruction beyond the blast site. Several buildings nearby were engulfed in flames.

Men and women wailed and ambulances rushed to the scene where residents tried to find victims buried in the rubble of collapsed buildings. The blast left a crater that was 2 yards wide and more than 4 feet deep.

“I heard a huge blast. I saw flames,” said Syed Irfat Ali, a resident who described how people were crying and trying to run to safety.

A top government official, Taha Farooqi, said at least 37 people were confirmed dead and 141 more were wounded.

Sunni militant groups have stepped up attacks in the past year against Shiite Muslims who make up about 20 percent of Pakistan’s population of 180 million people. Sunni militants linked to al-Qaida and the Taliban view Shiites as heretics.

Tahira Begum, a relative of a blast victim, demanded the government take strict action against the attackers.

“Where is the government?” she asked during an interview with local Aaj News TV. “Terrorists roam free. No one dares to catch them.”

Two brazen attacks against a Shiite Hazara community in southwestern city of Quetta killed nearly 200 people since Jan 10. Pakistani militant group Lashkar-e-Jhangvi claimed responsibility for the bombings, which ripped through a billiard club and a market in areas populated by Hazaras, which are mostly Muslim Shiites.

Pakistan’s intelligence agencies helped nurture Sunni militant groups like Lashkar-e-Jhangvi in the 1980s and 1990s to counter a perceived threat from neighboring Iran, which is mostly Shiite. Pakistan banned Lashkar-e-Jhangvi in 2001, but the group continues to attack Shiites.

According to Human Rights Watch, more than 400 Shiites were killed last year in targeted attacks across the country, the worst year on record for anti-Shiite violence in Pakistan. The human rights group said more than 125 were killed in Baluchistan province. Most of them belonged to the Hazara community.

Human rights groups have accused the government of not doing enough to protect Shiites.

After the Jan 10 bombing, the Hazara community held protests, which spread to other parts of the country. The protesters refused to bury their dead for several days while demanding a military-led crackdown against the Lashkar-e-Jhanvi group. Pakistan’s president dismissed the provincial government and assigned a governor to run Baluchistan province.

No operation was launched against the militant group until another bombing in February killed 89 people.

The government then ordered a police operation and has said some members of the group have been arrested. One of the founders of the group, Malik Ishaq, was among those detained and officials said he could be questioned to determine if his group’s is linked to the latest violence against Shiites.